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how to change input voltage range and frequency of transformer

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Junior Member level 1
Sep 22, 2010
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Can anyone suggest how to design the transformer with operating voltage range 100-240VAC 50/60Hz output 5V/100mA. how to increase the operating range and frequency of transformer.

Thnx in advance

For a standard power transformer the induced voltage in the secondary winding is in proportion to the primary voltage, and is given by the ratio of the number of turns in the secondary to the number of turns in the primary. So, you need to change the ratio in order to get rated output voltage when the incoming voltage is different than the normal voltage. By placing a tap in the primary winding, we could change the turns ratio, to get the 5 volts output with 100 volts or 240 volts input.

Maybe you need to convert utility AC voltage into regulated 5 DC voltages , and in this case you need to design some sort of ''universal AC-DC adapter’’, the most usual solution to use a SMPS .
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I need to have wide input voltage range of 100 to 260V AC 50/60Hz support. what we need to consider for wide range of input voltage

A transformr needs multiple primary windings and a voltage selector switch to be suited for wide voltage range. Using a DPDT switch and a dual primary winding, you can achieve 115/230 V selection, which is sufficient for many applications.

Most "wide voltage range" supplies are however switched mode converters that can work without a selector.

50/60 Hz isn't an issue for transformers, a 50 Hz transformer works at 60 Hz as well (and even up to 400 Hz with reduced performance).

how to select transformer for our application, it should be very compact in size

thnk for reply

Depends on your available suppliers. I would check a catalog distributor's website.

You have to get clear about your specification, is 5V/100 mA the required AC rms output?

i am designing the power supply to drive 5V DC load of 100mA

Are you familiar with the SMPS? At 5V/0.1A please consider any ‘’universal’’ phone charger as an example for.

There’s also a possibility to use a classical transformer.
In order to have a large range of voltage on input, we need to calculate the transformer at maximum ratings, in this case 240Vac, but to compensate the ratio number of turns primary vs secondary to work also at 100Vac input.
This means start to consider the minimum voltage needed in the secondary for 100Vac input, in order to obtain a 5Vdc output.
Let’s assume to use a standard LM7805 series of three terminal positive regulators.
Minimum dropout voltage =3V, so we need a minimum 8Vdc on input for a 5Vdc on the output.
For the full wave rectified wave, we need on secondary minimum (8/1.4 + 1.2)= 7Vac
In this situation (100Vac input) the ratio
Np/Ns= 100/7= 14.28

Now to change for the situation we apply 240Vac on input.
For the chosen ratio in the secondary we obtain 240/14.28= 16.8Vac
After the rectifier bridge a maximum of (16.8*1.41)=24Vdc output voltage.
A LM7805 can work with maximum 35Vdc on input.
Power dissipated by the integrated circuit
P=(24-5)*0.1= 1.9W, that’s OK if adequate heat sinking is provided.
To compensate power losses in the transformer a minimum 4VA is required.

Empirical solution:
4VA transformer design to work on 240Vac, with 16.8Vac on secondary will do the job.

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